Currently, around 11 million people are diagnosed as tumor patients every year worldwide, and it is speculated that this number will increase to more than 16 million by the year of 2020. In 2005, among the 58 million deaths, 7.6 million are caused by cancer (accounting for about 13%). This number is increasing, and it is expected that 9 and 11.4 million people will die of cancer by the year of 2015 and 2030 respectively. (World Health Organization, 2006)
Tumor markers are the substances produced by tumor cells during the progression caused by gene mutation, including antigens and other bio-active substances, which can be used for the early detection of cancers as well as the monitoring of disease progression and response to a treatment (ASCO, 1996). It brings huge benefit for the clinical treatment of cancers, especially when it can be detected before any obvious clinical phenomenon or when it can be used to monitor the patients' response to certain treatment. At present, in order to better meet the clinical need, greater efforts on the research and development of tumor biomarkers are required.
The applications of most tumor biomarkers currently used in clinic are more or less restricted due to the not-so-good sensitivity and specificity. For example, the AFP level and ultra-sonic examinations are largely used for liver cancer detection. Although their sensitivities are not very high, they indeed prolong the survival rate of the patients by diagnosis of the high-risk people. The tumor antigen CA-125 has a higher sensitivity but lacks specificity. Similarly, the blood tumor biomarker CA15-3 which is used for the detection of breast cancer could hardly be used for early detection due to low sensitivity. Therefore, methods for the early detection of cancer as well as to distinguish benign and malignant tumors are currently not available in clinic. New technologies as well as new methods are required to be developed to resolve these problems.
The development of tumor proteonomics brings hope for the identification of novel tumor biomarkers. The malignant transformation of tumors always results in the change of protein expressions, which could be quantified at the protein level. Thus, a lot of information and data could be derived, by which potential biomarkers could be identified and evaluated for further development and clinical application.
Hsp90α (Heat shock protein 90α, Hsp90α) is a molecular chaperone, which functions to stabilize its client proteins in their active states. Hsp90α is one of the most abundant proteins in the eukaryotic cells accounting for about 1-2% of whole cell proteins. The intracellular Hsp90α mainly functions to stabilize its clients (i.e. estrogen receptor) and assistant their maturation (i.e. some kinases and signal proteins). However, in other physiological conditions, Hsp90α is also involved in mediating events such as the evolution of mutated proteins, rearrangement of cytoskeleton, translocation of nuclear proteins, cell proliferation and apoptosis, protein degradation, antigen processing and LPS recognition etc. Hsp90α is also related with many diseases such as cancer, autoimmune disorder and cardiovascular diseases. For example, the monoclonal antibody against the antigen of LKVIRK sequence derived from Hsp90α can be used to treat fungal-related infection, and this clinic trial is currently ongoing by the Neutec company (Trade name: Mycogrip).
It is also reported that Hsp90α could be secreted under some stimulus (Liao et al. (2000) J. Biol. Chem. 275, 189-96). As a classical intracellular protein, there is little report regarding the function of extracelluar Hsp90α. In previous reports, Hsp90α was identified to help the antigen processing in APCs and was one of the four proteins related to the lipid raft. They can interact with LPC thus trigger the intracellular response of cells. (Triantafilou et al. (2002) Trends in Immunology 23, 301-4).
Hsp90α was also found to be highly expressed in the surface of some tumor cells, including the small-cell-lung cancer cell, melanoma and liver cancer cells (Ferrarini et al. (1992) Int. J. Cancer 51, 613-19). The high expression of cell surface Hsp90α in these cells were speculated to be related with the antigen processing while direct evidence is not available.
It is also reported that Hsp90α could help the translocation of transmembrane proteins (Schlatter et al. (2002) Biochem. J. 362, 675-84), and is related with the efflux of some anti-leukemia, lung cancer, cervix cancer drugs (Rappa et al (2002) Oncol. Res. 12, 113-9 and Rappa et al (2000) Anticancer Drug Des 15, 127-34).
The intracellular Hsp90α is an important target for the development of anti-cancer drugs, as it is involved in the regulation of many signaling pathways which are critical for the cancer cell transformation. Inhibition of intracellular Hsp90α could result in the selective degradation of proteins related with cell proliferation, cell cycle control as well as apoptosis. Recently, some known antibiotics such as Geldanamycin, Radicicol and Coumermycin A1 are natural inhibitors of Hsp90α. A patent (WO 00/53169) describes this mechanism and proposes that preventing the interaction of chaperones with its clients could result in the inhibition of its chaperone activity. Among these antibiotics, Coumarin and its derivatives are believed to have this activity. However, these inhibitors described in patent WO 00/53169 mainly target the intracellular Hsp90α.
The analogue of Geldanamycin 17-AAG is also an inhibitor of Hsp90α and is currently under clinical trials. However, some reports show that 17-AAG could have non-specific inhibitory effects and cell toxicity by interacting with many other cellular components. In addition, due to the limited knowledge on the physiological functions of Hsp90α and its clients, direct inhibition of intracellular Hsp90α is risky.
The patent (EP1457499A1) describes the function of extracellular Hsp90α in promoting tumor cell invasion via activating the MMP-2. Based on these mechanisms, the patent proposes that inhibition of extracellular Hsp90α could prevent the tumor invasion and metastasis, and by detecting the response of tumor cells to the treatment of Hsp90α inhibitor they can deduce the invasive ability of the cells and their relationship with Hsp90α.
The inventors of patent WO/2008/070472 propose that they can monitor the anti-tumor efficacy of Hsp90α targeted therapy by detecting the plasma Hsp90α and other related factors. In this patent, they provide the relationship between the plasma Hsp90α level and the efficacy of the inhibitors including 17-AAG and 17-DMAG as well as the relationship between the level of plasma Hsp90α and tumor volume in mouse models. However, they do not provide any evidence about the exact form of plasma Hsp90α and do not demonstrate the relationship between the plasma Hsp90α level and tumor malignancy especially tumor metastasis. They do not propose the application of plasma Hsp90α as an independent tumor biomarker in tumor diagnosis and prognosis, either.
One group reported that serum Hsp90α level is related with the stages of non-small-cell lung cancer (Xu et al. (2007) J. Cancer Mol. 3, 107-112). The serum level of Hsp90α in these lung cancer patients was significantly higher than that of normal people or benign tumor patients. However, again this paper did not identify the exact form of serum Hsp90α as well as its relationship with tumor metastasis. Besides, it only investigated non-small-cell lung cancer, while the relationship between serum Hsp90α level with breast cancer, liver cancer, and pancreatic cancer is unknown. Moreover, the serum level of Hsp90α was not quantitatively measured, thus could hardly be translated to clinic development and further application.